In recent years, magnetic disk drives have been increasing in capacity and thus in track density. With the increased track density, there have been growing demands for accurate head positioning. For accurate head positioning, the response speed in head positioning control needs to be improved, that is, the control frequency band needs to be shifted in a frequency increasing direction. Thus, magnetic disk drives have recently been proposed which comprise not only a VCM actuator but also a microactuator that is suitably follows high frequencies, that is, magnetic disk drives with a dual stage actuator (DSA) structure applied thereto.
The VCM actuator is driven by a voice coil motor (VCM). The VCM drives the VCM actuator when supplied with a current. That is, the VCM actuator is of a current driven type. In contrast, the microactuator is driven by applying a voltage to elements (for example, piezoelectric elements) forming the microactuator. That is, the microactuator is of a voltage driven type. Thus, the VCM actuator and the microactuator are different in driving method.
As a factor that affects the head positioning accuracy, disturbances such as vibrations and impacts to which the magnetic disk drive may be subjected are known. When the magnetic disk drive is subjected to such disturbances, the VCM actuator also vibrates, thus reducing the head positioning accuracy. Hence, to accurately position the head, disturbance compensation is required to suppress the adverse effect of the disturbance on the head positioning.
In general, feedback control is used for sudden disturbances such as vibrations or impacts. However, in an environment in which a severe disturbance is likely to occur, the disturbance resistance (vibration suppression) offered solely by the feedback control may be insufficient. Thus, feedforward control is applied to a steady-state disturbance such as runout.
When the VCM actuator is vibrated by the disturbance, acceleration occurs in the VCM actuator. Thus, in the conventional art, the acceleration of the VCM actuator is used for the feedforward control for compensating for the disturbance such as runout.
Disturbance feedforward control based on the acceleration is suitable for the current-driven VCM actuator. However, it is difficult to apply the disturbance feedforward control based on the acceleration to the voltage-driven microactuator. This is because the relationship between the acceleration and the voltage to be applied to the microactuator cannot be approximated by a linear expression.